In my copending application Ser. No. 603,776, filed Aug. 11, 1975, and entitled "Polyphonic Tone Synthesizer", there is described a musical instrument in which musical notes are generated in response to operation of a conventional keyboard. The waveform of each note is controlled by data calculated and stored in a master data list, the data being converted by a digital-to-analog converter to an analog waveform. The fundamental frequency of the waveform is determined by a voltage-controlled oscillator, the input voltage in turn being determined by the addressing of a frequency control number from the frequency number table. The particular key on the keyboard determines the address of the frequency number in the table, in a manner more specifically described in copending application Ser. No. 634,533, filed Nov. 24, 1975, and entitled "Frequency Number Controlled Clocks".
The present invention is directed to an improvement in the musical instrument described in the above-identified copending applications by which a portamento effect can be produced. The portamento effect is characterized by the sliding of the pitch of each note in a smooth transition to the pitch of the next note as successive notes are played on the keyboard. The present invention provides a transition which takes place over a time interval which is independent of the frequency of the notes. The transition takes place in a fixed number of incremental steps of equal frequency change, the change in frequency in each incremental step being a fixed fraction of the total frequency change in going from one note to the next. The total transition time can be adjusted to provide a slow or fast transition. Playing a succession of notes in a time less than the normal transition time does not interrupt or break the smooth frequency transition since the transition sequence begins at the same frequency present when the last key in the sequence is activated, even though the frequency has not reached the value of the previously executed key in the sequence.